Establishment and relief of CpG-dependent transgene repression during germ line passage and mouse development

Methylation of genomic DNA at CpG sequences has a repressive effect on gene expression in vertebrates. The level of methylation of the genome varies widely during development in mammals. The DNA of early germ cells and blastocysts is largely hypomethylated but that of implantation embryos is hypermethylated. To test whether these variations might affect gene expression, we have studied the pattern of expression of a CpG-rich and of a CpG-poor LacZ transgene driven by a strong promotor of an ubiquitous gene. We find that the changes of the expression of the CpG-rich LacZ gene directly correlate with variations of methylation and that lowering the level of CpG in the reporter gene decreases dramatically its susceptibility to these variations. Therefore, in association with fluctuations in genome methylation a CpG-dependent system of repression must control positively and negatively gene expression during development and gametogenesis. We also find that the repression of the CpG-rich LacZ reporter can be abolished in 2-cell embryo by inhibitors of histone deacetylases suggesting that, at least at this stage, this CpG-dependent repression acts through histone deacetylation. In addition, we show that the β-globin locus control region and other genomic elements completely reverse the repression established at implantation in embryonic cells. The complex patterns of expression of the CpG-rich and poor LacZ reporter genes during gametogenesis and development-including a sex-dependent expression in the zygotic nucleus before the morula stage-are described. The possible implication of the constraints imposed on gene expression by DNA methylation revealed by this study are discussed